1. Field of the Invention
The present invention relates to a pellicle, a method of preparing the same and an exposure method. More specifically, the invention relates to a pellicle suited for the lithography using ultraviolet rays and, particularly, vacuum ultraviolet rays, and a method of preparing the same. The invention is further related to an exposure method by using this particular pellicle.
2. Disclosure of the Prior Art
In the step of photolithography, the operation is carried out to transfer by exposure a circuit pattern onto a silicon wafer coated with a resist by using a photomask or a reticle (hereinafter referred to as xe2x80x9cmaskxe2x80x9d) having the circuit pattern which comprises a deposited film such as of chromium on the surface of a glass plate or by using a reticle. In this step, when the exposure is conducted in a state where foreign matter such as dust is adhered on the circuit pattern of the mask, the foreign matter is transferred onto the wafer resulting in the formation of a defective wafer which is a defective product. When the exposure is effected by using a stepper, in particular, the chips formed on the wafer may all become defective. Therefore, adhesion of foreign matter on the circuit pattern of the mask arouses a serious problem. In order to solve this problem, a pellicle has been developed and has been contrived in a variety of ways.
A pellicle, in general, is obtained by lining one side surface of a pellicle frame made of aluminum or the like with a transparent film of a resin such as nitrocellulose or the like, and is mounted on a mask by applying an adhesive to the other side surface thereof. This prevents the infiltration of foreign matter from the external side. Besides, even if foreign matter happens to adhere on the pellicle film, foreign matter is transferred in a blurred state by the exposure without arousing problem.
In the processing of semiconductors, it is a tendency to use a source of light having a short wavelength in order to enhance the degree of integration by forming fine patterns. It is expected that the sources of light in the vacuum ultraviolet region (xcexxe2x89xa6200 nm) are most promising and among them, an ArF excimer laser (xcex=193 nm) is most promising to succeed the KrF excimer laser (xcex=248 nm) of which the market is now on the rise.
As the wavelength of the source of light becomes short, on the other hand, the energy of photon increases. For example, the ArF excimer laser possesses the energy of as large as 6.4 eV (=147 kcal/mol). This energy is very larger than the dissociation energy (104 kcal/mol) of the Cxe2x80x94C bond in the organic polymer. Therefore, the polymer that absorbs light at the wavelength of the source of light easily undergoes the photolysis upon exposure to light and can no longer be used as a material of pellicle film.
On and after using the KrF lithography, therefore, there have been used fluorine-contained polymers that absorb light in relatively small amounts in the deep ultraviolet regions, such as commercially available fluorine-contained resin CYTOP (trade name) manufactured by Asahi Glass Co. and fluorine-contained resin Teflon (trade name) manufactured by du Pont Co., U.S.A.
However, even these fluorine-contained polymers absorb light in the vacuum ultraviolet region, and the thickness of the film decreases due to photolysis.
For example, the fluorine-contained resin CYTOP (film thickness of 1 xcexcm) absorbs about 0.5% of an ArF excimer laser beam of an oscillation wavelength of 193 nm, and loses light resistance to a large extent compared with when it is irradiated with the KrF excimer laser beam.
The object of the present invention therefore is to provide a pellicle that excellently transmits ultraviolet rays and, particularly, vacuum ultraviolet rays, does not lose the film thickness thereof that stems from the photolysis and, hence, exhibits excellent light resistance, and a method of producing the same.
Another object of the present invention is to provide an exposure method, which, even when ultraviolet rays are used and, particularly, vacuum ultraviolet rays are used, does not cause the pellicle to lose light resistance that stems from the photolysis and, hence, makes it possible to form vivid and fine patterns by the lithography for relatively extended periods of time.
According to the present invention, there is provided a pellicle film obtained by using, as a material of pellicle film, an impurity-free organic polymer obtained by treating an organic polymer to remove at least a portion of or, preferably, a majority portion of at least any one of trace amount-metal components, high molecular components and incomplete molecular structure components contained in the organic polymer.
According to the present invention, there is further provided a method of preparing a pellicle film by conducting the operation for obtaining an impurity-free organic polymer by removing at least a portion of or, preferably, a majority portion of at least any one of trace amount-metal components, high molecular components and incomplete molecular structure components contained in the organic polymer, by using a filter that exhibits the adsorbing action based on the zeta potential.
According to the present invention, there is further provided a pellicle film in which the contents of trace amount-metal components in the impurity-free organic polymer are not larger than 1 ppm, respectively.
According to the present invention, it is desired that the organic polymer is a fluorine-contained resin comprising carbon (C) and fluorine (F) as chief constituent components and, particularly, is a fluorine-contained resin comprising carbon (C), fluorine (F) and oxygen (O) only as chief constituent components.
According to the present invention, there is provided an exposure method by using the pellicle film in the lithography that uses a source of light for exposure to ultraviolet rays having wavelengths over a range of from 140 to 200 nm.
As an embodiment, the invention is further concerned with a pellicle film comprising a fluorine-contained resin which contains carbon (C) and fluorine (F) as chief constituent components, the fluorine resin being treated with a soluble solvent.
It is desired that the fluorine-contained resin treated with the soluble solvent is obtained by isolating part of the dissolved fluorine-contained resin from the solution of a soluble solvent of the fluorine-contained resin in the form of a solution and/or being precipitated.
As a preferred embodiment, the thus obtained fluorine-contained resin contains metal components in amounts of not larger than 1 ppm, respectively.
It is further desired that the fluorine-contained resin used in this embodiment is treated with a soluble solvent to remove at least part of high molecular components from the fluorine-contained resin.
The pellicle films obtained by the above-mentioned various embodiments are suppressed from being subjected to the photolysis even when they are used for the lithography using ultraviolet rays as a source of light for exposure, and exhibit durability that could not be obtained so far.
That is, another embodiment of the present invention is to provide a novel pellicle film simultaneously satisfying the following conditions (a) to (c):
(a) when an ArF excimer laser beam (xcex=193 nm) is irradiated under the following conditions, the total dosage before the thickness of the film is decreased by 5 nanometers (nm) is not smaller than 1420 joules/square centimeter (J/cm2), preferably, from 1420 to 28400 joules/square centimeter (J/cm2) and, particularly preferably, from 1420 to 14200 joules/square centimeter (J/cm2),
ArF excimer laser beam irradiation conditions:
(b) the pellicle film comprises a fluorine-contained resin comprising carbon (C) and fluorine (F), or further oxygen (O), as chief constituent components; and
(c) the pellicle film has a thickness of from 0.1 to 10 microns (xcexcm).
These pellicle films are suited for the exposure method in the lithography using a source of light for exposure to ultraviolet rays having wavelengths over a range of from 140 to 200 nanometers (nm), and exhibit excellent light resistance that was not obtained so far.
The pellicle film of the present invention contains impurities in decreased amounts, exhibits excellent transmission property for the ultraviolet rays and particularly for the vacuum ultraviolet rays, suppresses the reduction in the thickness caused by the photolysis, and exhibits excellent light resistance.
The method of the present invention removes impurities that cause a drop in the ultraviolet ray transmission through a simple operation of passing the resin solution for forming the film through a filter that exhibits the adsorbing action based on the zeta potential or by effecting the precipitation using a soluble solvent.
According to the present invention, further, the pellicle little loses light resistance caused by the photolysis even when ultraviolet rays and, particularly, vacuum ultraviolet rays are used, enabling sharp and fine patterns to be formed by lithography for relatively long periods of time.